Frequency shift discriminator



July 22, 1958 E. A. GILBERT FREQUENCY SHIFT DISCRIMINATOR Filed Dec. 6, 1954 S u? L A OMT* United States Patent FREQUENCY srnrr nlscnrMrNAroR Everett A. Gilbert, Denville, N. J., assignor to Radio Frequency Laboratories, Inc., Boonton, N. Si., a corporation of New Jersey Application December 6, 1954, Serial No. 473,091

11 Claims. (Cl. Z50-27) This invention relates to a frequency shift discriminator and more particularly to an electronic circuit for converting variations in the frequency of an input signal to variations in a D.-C. output voltage.

Frequency shift discriminators are well known and nd general application in the fields of communications and controls. A discriminator circuit made in accordance with this invention is of general utility but it is particularly suitable for converting frequency shift audio signals to D.-C. voltage pulses in a teleprinter of the type disclosed in United States Patent No. 2,654,025 issued to Charles A. Higgins on September 29, 1953. In teleprinters of this class, it is customary to transmit coded radio waves of two different frequencies in alternate sequence, each succession of wave pulses having a precise time duration measured, generally, in milli-seconds. Various characters, figures, punctuation marks, etc. are represented by individual combinations of the two wave pulses in accordance with the standard radiotype code and these coded sequences are translated at the receiverconverter into direct current pulses of corresponding time duration. Such direct current pulses are utilized to set up an electromagnetic selector mechanism preparatory to the imprinting of the corresponding character, etc. on a sheet of paper. Present communication systems of this class are known as two frequency systems operating in the radio spectrum say, 2 megacycles, with a frequency separation of 850 cycles. However, the most useful application of the device is in telegraph and control applications requiring the use of audio frequencies over voice circuits. Thus, a given character to be transmitted will comprise a precisely-timed sequence of wave pulses formed of the two frequencies. Itis common practice to designate one of the frequencies as a Space signal and the other as a Mark signal. The receiver includes a frequency shift discriminator type of demodulator to distinguish between the Mark and Space signals and to produce corresponding D.C. pulses. Only the Mark signal pulses are effective to cause actuation of the selector magnet mechanism of the teleprinter. Since the D.-C. pulses actuating the selector magnet mechanism vary in time duration and spacing in accordance with the transmitted-received radio waves the selector mechanism is set up properly for the power operation of the type bar corresponding to the particular character. The present invention is directed to the provision of a novel discriminator for converting the frequency-shift incoming signals to corresponding D.C. pulses.

An object of this invention is the provision of apparatus for converting a variation in the frequency of an applied signal into corresponding D.C. pulses and which apparatus is useful for carrier telegraph and tone control applications.

An object of this invention is the provision of a frequency shift discriminator circuit that is very sensitive to changes in the frequency of an applied signal and 2,844,720 Patented July 22, 19,58

which provides a positive D.C. output voltage when no signal is applied thereto.

An object of this invention is the provision of a frequency-shift discriminator comprising an electron tube having a plurality of control grids, a rst circuit including a reactance and developing a voltage that is displaced 9G degrees with respect to that of an applied signal, circuit elements to impress the said voltage on one of the control grids, a tuned circuit energized by a voltage that is in phase with the applied signal, and means impressing the voltage across said tuned circuit on the other control grid of the tube.

An object of this invention is the provision of apparatus providing a D.-C. voltage output which varies in accordance with variations in the frequency of an applied signal comprising a first electron tube having two control grids and an anode connected to a source of biasing voltage through a load resistor; a second electron tube having a control grid, a cathode and an anode; a resistor and capacitor connected in parallel and between the anode of the second tube and a source of biasing voltage; circuit elements impressing the voltage appearing across said capacitor to one control grid of the said first tube; means impressing an incoming signal across the grid and cathode of the second tube; a resistor in the cathode circuit of the second tube; a tuned circuit connected across the said resistor in the cathode circuit of the second tube; means impressing the voltage across said tuned circuit on the other control grid of the said first tube; means supplying a predetermined quantity of energy into the tuned circuit; and voltage-responsive means responsive to the voltage drop across the load resistor in the anode circuit of the first tube.

These and other objects and advantages will become apparent from the following description when taken with the accompanying drawings in which:

Figure l is a circuit diagram of apparatus made in accordance with my invention; and

Figure 2 is a vector diagram showing the phase relationship of the various voltages applied to the control grids of the discriminator tube.

Reference is now made to the circuit diagram of Figure l. An incoming signal is applied to the input terminals i0 and the carrier frequency is separated from the common bus by a suitable band pass filter 11. The passed signal is impressed on the control grid of a double triode vacuum tube 13, which, together with the associated circuitry, constitutes a limiting amplier. The ouput signal of the amplifier is applied to the control grid of the electron tube i4, which is arranged as a cathode follower and phase shifter with the impedance of the load capacitor l5 (as an example of a phase-shifting impedance) approximately 10,000 ohms at the carrier frequency. This capacitor is in parallelA with the biasing resistor 39 in series between the positive side of the 150 volt supply and the anode 40. The grid 38 is biased by resistor 4l. Thus, the voltage E2, across the capacitor 15, leads the voltage El at the cathode by degrees. This voltage, El, is applied through a D.C. blocking capacitor i6 and a driving resistor 17, to a tuned circuit 18 comprising the capacitor 19 and inductance coil 20.

In the audio range, the Q of the inductance coil 20 generally is not sufciently high for narrow band operation. In order to increase the Q of the coil, and correspondingly the overall sensitivity of the circuit, the electron tube 2l is arranged to regeneratively feed back energy to the tuned circuit. The amount of energy so fed back is controlled by the value of the resistor 22 and such resistor is adjusted at the factory to a value which prevents oscillation o-f the tube.

The voltages E2 (across the capacitor 15 in the plate circuit of the tube 14) and El (across the cathode biasing series resistor 23 of the tube 21) are respectively applied to the control grids 24 and 2S of the tube 26, the anode 35 of which is connected to the positive side of a 150 volt supply through resistor 36. The latter tube is a gated beam type 6BN6 which has a high trans-conductance between each control grid and the anode. Consequently, both grids exercise a high degree of control over the flow of the electron stream through the tube. Each grid can switch the plate current between cut off and its limited value with only a relatively low applied grid signal. When alternating current signals are applied to the co-ntrol grids 24, 25 each grid will tend to throw the tube into a condition of complete cut off or complete conduction in accordance with the instantaneous signal polarities. inasmuch as either grid can cut off the ow of plate current the tube will conduct only during that portion of each cycle for which the two grids are positive at the same time. By varying the relative phase of the grid signals the length of time that the tube conducts can be varied.

At the selected center frequency of the circuit, that is, at the resonant frequency of the tuned circuit 18, the voltage E1 applied to the grid 25 of the tube 26 through resistor 37 is in phase with the voltage El developed across the cathode biasing resistor 27 of the tube 14, since under this condition the tuned circuit is resistive. When the applied signal frequency, f, is less than the resonant frequency, fc, of the tuned circuit the latter effectively becomes inductive and the voltage El' leads E1. Conversely, when the signal frequency is greater than fo the tuned circuit effectively becomes capacitive and the voltage E1 lags El. These relative phase relationships are shown in the vector diagram of Figure 2, it here being pointed out that the voltage E2 applied to the other control grid 24 retains its 90 degree leading character relative to the voltage El' applied to the rst grid under center band frequency conditions. It will be apparent that when the frequency, f, of the input signal is less than the resonant frequency, fo, of the tuned circuit, the voltages E2 and El applied to the two control grids of the tube 26 are relatively in phase and a maximum plate current will ow in the tube anode circuit. Conversely, when the input signal frequency is above the resonant frequency of the tuned circuit the voltage El becomes out of phase with the voltage E2 whereby the two grids operate in an alternate manner to keep the ow of anode current cut off at all times. The output voltage of the tube 26 is applied to the control grid yof an amplifier tube 28 through a resistive coupling network 29, connected to the negative side of the 150 volt supply, the plate load of such tube being the operating coil of a power relay 3i). A capacitor 31, interposed between the anode of the tube 26 and ground, serves to filter the audio carrier frequency from the low frequency telegraph or control signal.

It will now be apparent that the circuit will effect the actuation of the power relay when the frequency of the incoming, applied, signal is lower than the selected center band frequency of the circuit and that actuation of the relay may be converted into visual signals and/or a control function by means of suitable circuits controlled by the relay contacts. As stated hereinabove, when the circuit is used in connection with a teleprinter only the Mark signals are utilized to effectuate operation of the printer selector magnets. It is general practice to provide a continuous Mark-Hold signal when no input signal is received by the receiving apparatus. For this. latter purpose I have adjusted the value of the resistor 32, in the cathode circuit of the discriminator tube 26, so that when no signal is applied to the circuit input terminals 10, the current through the screen grid 33, its series resistor 42, and the voltage dropping resistor 34 will completely block plate conduction of the tube, thereby removing the-normal negative bias of about 30 volts, due to the connection through network 29 with the voltage CTI supply, and applying a positive signal to the control grid of the amplifier tube 28 resulting in the energization of the operating coil of the relay 30.

Having now given a detailed description of my invention in accordance with the requirements of the patent statutes, what I desire to protect by Letters Patent of the United States is set forth in the following claims.

I claim:

l. Apparatus for producing a variation in a D.C. output voltage in accordance with the frequency variation of an applied signal voltage from a predetermined reference frequency, said apparatus comprising an electron tube having a cathode, an anode and two control grids, a D.-C. voltage source connected to the anode through a resistor, means impressing a first voltage on one of the' control grids which voltage is displaced substantially degrees relative to the applied signal voltage, a tuned circuit .resonant at the said predetermined reference frequency, and energized by the signal voltage, circuit ele-ments impressing the voltage across the tuned circuit to the second control grid of the tube, and means regeneratively feeding energy into the tuned circuit to compensate for its losses.

2. Apparatus for producing a variation in a D.C. output voltage in accordance with a variation in the frequency of an applied `signal from a predetermined reference frequency, ksaid apparatus comprising a first electron tube having a cathode, a grid and an anode; a resistor in the tube cathode circuit, a load resistor shunted Vby a reactive component in the tube plate circuit; means impressing the applied signal between the grid land cathode Voffthe said first tube; a second electron tube `having Va cathode, a grid and an anode; a resistor in vtheicathode circuit of the second tube; a lead connecting the grid of the second tube to the cathode of the first tube through a capacitor; a tuned circuit connected across the resistor in the cathode circuit of the first tube, said circuit being resonant at the reference frequency; a regenerative feedback connection between the vtuned 'circuit and the cathode of the second tube; a third electron `tube yhaving a cathode and anode and two control grids; -c-ircuitelements applying relative to ground the voltage across the cathode resistor of ythe second tube to one -of the control grids of the third tube; circuit elements applying the voltage across the said reactive component to the other control grid of the third tube; -and 1a load resistor in the anode circuit of the said third tube.

3. The Vinvention as Vrecited in claim 2 wherein the said third tube vincludes a screen grid; a first resistor connected across the said load resistor and the screen grid; and a secondzresistor connected across said load resistor and the cathode.

4. Apparatus for producing a variation in a D.C. output voltage in response to a frequency variation in an applied. signal from a predetermined reference frequency, said apparatus comprising a first electron tube having a cathode, -agrid vand an anode, rst and second resistors connected to the anode and cathode, respectively, of the said first electron tube, a reactive member in parallel with the said first resistor, means impressing the applied signal onthe control grid of the said rst tube, a tuned circuit resonant at the reference frequency and energizedby the alternating signal voltage developed at the said second resistor, a second electron tube having a cathode, a grid and an anode, a third resistor connected to the said cathode of the second tube, means impressing the signal developed across the tuned circuit on the said grid `of the second tube, a regenerative feedback circuit between the tuned circuit and the cathode of said second tube, a third electron tube having a cathode, an anode vand two control grids, circuit elements impressing the voltage relative to ground across the said reactive member on one ofthe control grids of the third tube, means impressing the voltage developed across the third resistor on the other control grid of the third tube, and a load resistor in the anode circuit of the third tube.

5. The invention as recited in claim 4, wherein the said third tube includes a third grid; a fourth resistor connected across the said load resistor and the third grid, and a fth resistor connected across the said load resistor and the cathode.

6. Apparatus for producing a variation in a D.C. output voltage in accordance with the frequency variation of an applied signal voltage from a predetermined reference frequency, said apparatus comprising an electron tube having a cathode, an anode and two control grids, a D.C. voltage source connectedto the anode through a resistor, means impressing a first voltage on one of the control grids which voltage is displaced a constant phase angle of substantially .90 degrees relative to the applied signal voltage over the frequency range of the applied signal voltage, a parallel tuned circuit sharply resonant at a predetermined center frequency and energized by a potential in phase with the applied signal voltage, and means impressing the voltage across the parallel tuned circuit to the second control grid of the tube.

7. The invention as recited in claim 6, wherein the said parallel tuned circuit comprises a parallel connected capacitor and inductor.

8. The invention as recited in claim 6, including means regeneratively feeding energy into the tuned circuit to compensate for its losses.

9. Apparatus for producing a variation in a D.C. output voltage in accordance with the frequency variation of an applied signal voltage from a predetermined reference frequency, said apparatus comprising an electron tube having a cathode, an anode and two control grids, a D.C. voltage source connected to the anode through a resistor, a rst circuit path including a reactive element energized by the applied signal voltage, means deriving a first voltage from the current flowing in the said reactive element, means applying the said lirst voltage to one of the control grids, a second circuit path including a parallel resonant circuit sharply resonant at the said reference frequency and energized by the applied signal voltage, means deriving a second voltage from the current owing in the said parallel resonant circuit, and means applying the said second voltage to the second control grid of the tube.

l0. The invention as recited in claim 9, wherein the said parallel resonant circuit comprises a parallel connected capacitor and inductor.

11. The invention as recited in claim 9, including means regeneratively feeding energy into the parallel resonant circuit to compensate for its losses.

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